Egress Traffic Concepts - Allied Telesis SwitchBlade x3100 Series Manual

Introduction
For IP packets there are the DSCP field and the TOS field. The COS identifier can specify both a service
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level priority and the precedence for dropping packets, but this is not done at the ethernet level.
Once a packet is marked, it may go immediately to a COS queue, but in many cases the traffic is metered. To
meter the packet flow is to monitor or police the rate of traffic flow and to see if incoming traffic exceeds the
bandwidth specified in the SLA. When packet flow exceeds the bandwidth allocated, they are labeled as Out Of
Profile (OOP). This policing function is done using the leaky bucket algorithm. The bucket has a capacity and
an output rate as packets enter and leave the bucket.
If packets arrive at a rate faster than contracted for in the SLA for a continuous period of time, the bucket will
overflow. These overflow packets are classified as out of profile and another action can be applied to them, such
as drop (throw away), or remark them in such a way that these OOP packets have a higher probability of
being thrown away when congestion points are encountered through the device or further on in the network.
Packets that exceed this bandwidth are labeled as out of profile with the SLA.
By metering the rate at which packets arrive, the provider can control bandwidth, since the SLA may include a
minimum bandwidth availability and as well as a maximum (for short periods). These are defined as follows:
Committed Information Rate (CIR) is the minimum guaranteed rate the provider network will provide
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under normal conditions, and is measured in bits per second.
Any service that provides a non-zero bandwidth guarantee must have a CIR. A CIR of zero indicates
Note:
the service will provide no minimum guarantee for frame delivery.
Committed Burst Size (CBS) is the maximum number of bytes that can be sent at the CIR and is measured
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in kilobytes (KB) or megabytes (MB)
Peak Information Rate (PIR) is the maximum rate at which frames/packets are allowed to burst above the
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CIR and is measured in bytes per second.
Peak Burst Size (PBS) is the maximum number of bytes that can be sent at the PIR and is measured in kilo-
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bytes (KB) or megabytes (MB).
For a service that requires only a CIR and CBS, a single meter is used; the capacity of a single bucket is equal to
the CBS and the leak rate is the CIR. For services that require all four parameters, two meters are required.

6.2.2 Egress Traffic Concepts

Once the traffic flows have passed through the policing function and are allowed to continue, the COS identi-
fier (obtained when the traffic was marked) is used to map the traffic with a set of queues and to assign a prior-
ity. Each queue is associated with a level of service of low versus high. If a QoS network provides four levels of
service, there will typically be four queues. Traffic flows will be associated with a priority (using the COS iden-
tifier bit) and therefore a queue.
As the packets are placed in the queues, there may still be conditions where packets may need to be dropped.
One method of handling overflow is tail-drop; when a queue is in an overflow state, all newly arriving packets
are dropped. If the potential for queue overflow was too high, the queue size(s) were increased. However,
dynamic managing of queue depth can result in better network performance, and one method is Random
Early Discard (RED), which improves network throughput and lowers the probability of packet discard.
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Software Reference for SwitchBlade x3100 Series Switches (Access and Security)
Egress Traffic Concepts